Pros and Cons of Illya?

She wouldnt really "replace" Tamamo. They have two very different rolls on a team. Tamamo is a support meanwhile Illya is a single target nuker. Illya pros are that she can dish out a lot of damage with her np especially against casters, and that she is an adorable loli. For the cons she has a few. She has trouble generating her NP with low gain and no skill to help with that, the fact that she has a "chance" skill means its not guaranteed, and since shes a caster she does very little damage other than her NP. I think shes a lot better than what some people make her out to be, but shes by no means one of the best. But she get points for being cute.

Let's be honest, Illya can't replace Tamamo. Tamamo is tier 2 while Illya is Tier 5. Pretty bad if you ask me.

Pros for sure ST Buster NP and Buster Increase Skill, Invincible Skill, she can survive things with her heal and guts (but again, guts is % based), really high NP damage.

Cons: Mediocre damage because of QAAAB deck and caster's 0.9x class modifier.
Really mediocre NP gain.
Selfish kit
And chance-based third skill.

They're different roles. Tamamo will never be expected to be a main dps but is an S+ tier support that makes art stall teams still relevant in the meta today. Illya on the other hand is a purely offensive boss killing nuke.

For illya,
Pros: Almost all her damage is tied to one extremely hard hitting nuke of an NP. With proper set up and support you can either one shot a boss in one turn with that np or at the very least do a butt load of damage. She also has an invincibility skill that ups her survivability in addition to an excellent self buster up.

Cons: Her np gain is really poor for a caster. Her np also has a 3 turn self debuff it puts on her attack and defense. She lacks reliable support spells for others which limits her versatility. The 0.9 class multiplier also means she's typically only useful with class advantage.

The main gripe about her is how reliant she is in that one thing to be useful. Generally servants that have to rely on one big burst of damage to be useful aren't the most reliable. She is a good servant for sure but doesn't stack up compared to say Xuanzang or Nero caster.

As all tier lists say, all 5 stars are worth levelling for the stats alone. I'd level her if you have time and embers and mats to spare because outside of the aforementioned 2, she's the best in her niche. Or you could just keep using berserkers

Like others have said, Illya and Tamamo might both be casters but they are both used for very different things. Tamamo is used for support while Illya is intended for damage.

Illya Pros: -Highest single target burst damage of all casters currently available.
-Targetable healing with chance for debuff immunity.

Illya Cons: -Lousy NP gain, even for a caster who gains 3% NP per turn.
-10% attack and defense demerit on NP
-5 star berserkers tend to outclass her in damage anyway.

Please don't take this the wrong way, because I'm not trying to be a jerk or anything, but did you know that this website has a list of all the servants, with pretty detailed writeups on most of them, and their pros and cons? You can either search for them using the search bar, or click on them from the tier list or servant list. Everything that has been mentioned in this thread, and more, is on Illya's page at https://grandorder.gamepress.gg/servant/illyasviel-von-einzbern

Once again, I'm not trying to be a smartass or anything. I'm just telling you in case you legitimately didn't know.